1. Field of the Invention
This invention generally relates to a signal transmission structure, and more particularly to a signal transmission structure for minimizing the power noise induced when signals pass through imperfect reference plane.
2. Description of Related Art
The signal line for connecting two devices or two terminals in a large-scale printed circuit board or a packaging substrate has to maintain a uniform line width in order to keep a constant characteristic impedance when an electronic signal is transmitted in the signal line. A good impedance matching design between two terminals is required to reduce the reflection due to the impedance mismatch especially when the signal is transmitted in a high speed and a high frequency environment. That is, the insertion loss should be reduced and the reduction of the return loss should be increased when transmitting the signals so that the quality of the signal transmission will not be affected.
FIG. 1 shows a cross-sectional view of a conventional signal line passing through a imperfect plane. The conventional signal transmission structure 100 includes a signal line 110, a first reference plane 120 and a second reference plane 130, sequentially. Wherein, the width of signal line 110 is uniform, and the first reference plane 120 and the second reference plane 130 are a power plane and a ground plane, respectively, for example. It is important to note that in a conventional circuit design, the first reference plane 120 can form a slot due to the hole drilling or cutting between the planes, or imperfect planes can be formed (such as a non-reference area opening 122) for avoiding short circuit in adjacent signal lines. As a result, a high impedance variation occurs at the non-reference area opening 122 when the signal is transmitted in the signal line 130 such that the impedance mismatch increases of the insertion loss. Therefore, the signal cannot be transmitted without a loss from one terminal of the non-reference area opening 122 to the other terminal of the non-reference area opening 122.
Meanwhile, when the signal passes by the non-reference opening 122, due to the sudden variation of voltage and current, the signal will stimulate the coupling of signal line 110 and power plane 130 and resonance behaviors of ground plane 120 and power plane 130, leading to power noise interference and deteriorating the quality of signal transmission.
In light of the above, the frequency increases, the return loss decreases and the corresponding characteristic impedance increases when the signal line passes through the imperfect reference area. Moreover, the difference in the characteristic impedance from the original design is increased such that the impedance mismatch in the signal lines is much more serious.